Not applicable
The present invention relates to laser trimming and, in particular, to laser trimming thick or thin film resistors with a uniform spot from a solid-state laser.
Conventional laser systems are typically employed for processing targets such as electrically resistive or conductive films of passive electrical component structures, such as film resistors, inductors, or capacitors, in circuits formed on ceramic or other substrates. Laser processing to trim the resistance values of film resistors may include passive, functional, or activated laser trimming techniques such as described in detail in U.S. Pat. No. 5,685,995 of Sun et al.
The following background is presented herein only by way of example to thick film resistors. FIG. 1 is an isometric view of a work piece 10, such as a prior art thick-film resistor 10a, forming part of a hybrid integrated circuit device, and FIG. 2 is a cross-sectional side elevation view depicting thick-film resistor 10a receiving a conventional laser output pulse 12. With reference to FIGS. 1 and 2, a conventional thick-film resistor 10a typically comprises a thick film layer 14 of a ruthanate or ruthinium oxide material extending between and deposited on portions of the top surfaces of metallic contacts 16. Layer 14 and metallic contacts 16 are supported upon a ceramic substrate 18, such as alumina. Modern ruthinium-based thick film pastes have been optimized to be stable after laser trimming with a 1.047 micron (xcexcm) Nd:YLF laser or a 1.064 xcexcm Nd:YAG laser.
With particular reference to FIG. 1, the resistance value of resistor 10a is largely a function of the resistivity of the resistor material and its geometry, including length 22, width 24, and height 26. Because they are difficult to screen to precise tolerances, thick-film resistors are intentionally screened to lower resistance than nomimal values and trimmed up to the desired values. Multiple resistors 10a having approximately the same resistance values are manufactured in relatively large batches and then subjected to trimming operations to remove incremental amounts of the resistor material until the resistance is increased to a desired value.
With particular reference to FIG. 2, one or more laser pulses 12 remove substantially the full height 26 of the resistor material within the spot dimensions 28 of laser output pulses 12, and overlapping spot dimensions 28 form a kerf 30. A simple or complex pattern can be trimmed through the resistor material of a resistor 10a to fine tune its resistance value. Laser pulses 12 are typically applied until resistor 10a meets a predetermined resistance value.
FIG. 3 is an isometric view of a portion of a prior art resistor 10 showing for convenience two common pattern trim paths 32 and 34 (separated by a broken line) between metal contacts 16. xe2x80x9cL-cutxe2x80x9d path 32 depicts a typical laser-induced modification. In an L-cut path 32, a first removal strip 36 of resistor material is removed in a direction perpendicular to a line between the contacts to make a coarse adjustment to the resistance value. Then an adjoining second removal strip 38, perpendicular to the first removal strip 36, may be removed to make a finer adjustment to the resistance value. A xe2x80x9cserpentine cutxe2x80x9d path 34 depicts another common type or laser adjustment. In a serpentine cut 34, resistor material is removed along removal strips 40 to increase the length of film path 42. Removal strips 40 are added until a desired resistance value is reached. Removal strips 36, 38, and 40 are typically the width of a single kerf 30 and represent the cumulative xe2x80x9cnibblingxe2x80x9d of a train of overlapping laser pulses 12 that remove nearly all of the resistor material within the prescribed patterns. Thus, when the trimming operation is completed, the kerfs 30 are xe2x80x9ccleanxe2x80x9d with their bottoms being substantially free of resistor material such that the substrate 18 is completely exposed. Unfortunately, the formation of conventional clean kerfs 30 necessitates a slight laser impingement of the surface of substrate 18.
As film resistors become smaller, such as in the newer 0402 and 0201 chip resistors, smaller spot sizes are needed. With the 1.047 xcexcm and 1.064 xcexcm laser wavelengths, obtaining smaller spot sizes while employing conventional optics and maintaining the standard working distance (needed to avoid ablation debris and to clear the probes) and adequate depth of field (ceramic, for example, is not flat) is an ever-increasing challenge. The desire for even more precise resistance values also drives the quest for tighter trim tolerances.
An article by Albin and Swenson, entitled xe2x80x9cLaser Resistance Trimming from the Measurement Point of View,xe2x80x9d IEEE Transactions on Parts, Hybrids, and Packaging; Vol. PHP-8, No. 2, June 1972, describes measurement issues and the advantages of using a solid-state laser for trimming thin film resistors.
Chapter 7 of an NEC instruction manual describes the challenges encountered when using an infrared (IR) Gaussian beam to trim resistors, particularly thick film resistors. Heat-affected zones (HAZ), cracks, and drift are some of the problems that are addressed.
An article by Swenson et al., entitled xe2x80x9cReducing Post Trim Drift of Thin Film Resistors by Optimizing YAG Laser Output Characteristics,xe2x80x9d IEEE Transactions on Components, Hybrids, and Manufacturing Technology; December 1978, describes using green (532 nm) solid-state laser Gaussian output for trimming thin film resistors to reduce HAZ and post trim drift.
U.S. Pat. Nos. 5,569,398, 5,685,995, and 5,808,272 of Sun and Swenson describe the use of nonconventional laser wavelengths, such as 1.3 xcexcm, to trim films or devices to avoid damage to the silicon substrate and/or reduce settling time during functional trimming.
International Publication No. WO 99/40591 of Sun and Swenson, published Aug. 12, 1999, introduces the concept of resistor trimming with an ultraviolet (UV) Gaussian laser output. With reference to FIG. 4, they employ the UV Gaussian laser output to ablate an area 44 of the surface of film resistors to maintain their surface area and conserve their high frequency response characteristics. By intentionally retaining a depth 46 of resistor film in the trimmed areas 44, they avoid having to clean the kerf bottoms 48 and substantially eliminate the interaction between the laser output and the substrate 18, thereby eliminating any problems that might be caused by such interaction. Unfortunately, surface ablation trimming is a relatively slow process because the laser parameters must be carefully attenuated and controlled to avoid complete removal of the resistor film.
Microcracking is another challenge associated with using a solid-state Gaussian laser beam for trimming resistors. Microcracks, which often occur in the center of a kerf 30 on the substrate, may extend into the resistor film causing potential drift problems. Microcracks can also cause a shift associated with the temperature coefficient of resistance (TCR). Such microcracking is more pronounced in the newer 0402 and 0201 chip resistors that are fabricated on thinner substrates 18, with a typical height or thickness of about 100 to 200 xcexcm, compared to those of traditional resistors. Microcracking in these thinner-substrate resistors can propagate and even result in catastrophic failure or physical breakage, particularly along the trim kerf 30, of the resistor during subsequent handling. Microcracking can also create xe2x80x9cpreferredxe2x80x9d break lines that are more pronounced than the desirable break prescribed break lines in snapstrates.
Improved resistor trimming techniques are, therefore, desirable.
An object of the invention is, therefore, to provide an improved system and/or method for solid-state laser trimming.
Another object of the invention is to provide spot sizes of less than 20 xcexcm to trim smaller chip resistors, such as 0402 and 0201 chips resistors.
Some of the microcracking may be caused by the high intensity center of the Gaussian beam spot in much the same way that a Gaussian beam may be responsible for damaging the center of a blind via in a laser drilling operation (although the targets and substrates are different materials). International Publication No. WO 00/73013 of Dunsky et al., published Dec. 7, 2000, describes a method for creating and employing an imaged shaped Gaussian beam to provide a uniform laser spot, particularly useful for via drilling operations.
An article by Swenson, Sun, and Dunsky, entitled xe2x80x9cLaser Machining in Electronics Manufacturing: A Historical Overview,xe2x80x9d SPIE""s 45th Annual Meeting, The international Symposium on Optical Science and Technology; Jul. 30-Aug. 4, 2000, describes an improved surface scanning method using a 40 xcexcm uniform spot formed by a lens described by Dickey et al. in U.S. Pat. No. 5,864,430.
The present invention preferably employs a uniform spot, such as an imaged shaped Gaussian spot or a clipped Gaussian spot, that is less than 20 xcexcm in diameter and imparts uniform energy across the bottom of a kerf 30, thereby minimizing the amount and severity of microcracking. Where appropriate, these spots can be generated in an ablative, nonthermal, UV laser wavelength to reduce the HAZ and/or shift in TCR. These techniques can be employed for both thin and thick film resistor processing.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.